I now answering my own question:)
ZaphodHeads is exactly what I was looking for.
I created 2 devices with Screen and ZaphodHeads, and it know works
Section "Monitor"
Identifier "External"
Option "PreferredMode" "1920x1200"
Option "RightOf" "Onboard"
EndSection
Section "Device"
Identifier "intelExt"
Driver "intel"
BusID "PCI:0:2:0"
Option "ZaphodHeads" "HDMI1"
Screen 1
EndSection
Section "Device"
Identifier "intelOnb"
Driver "intel"
BusID "PCI:0:2:0"
Option "ZaphodHeads" "LVDS1"
Screen 0
EndSection
Section "Screen"
Identifier "Onboard"
Monitor "Onboard"
Device "intelOnb"
SubSection "Display"
EndSubsection
EndSection
Section "Screen"
Identifier "External"
Monitor "External"
Device "intelExt"
SubSection "Display"
Modes "1920x1200"
EndSubsection
EndSection
Section "ServerLayout"
Identifier "DefaultLayout"
Screen 0 "Onboard"
Screen 1 "External"
EndSection
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